A desirable feature for vehicles is the provision of all-wheel drive. A vehicle's transmission engages the motor to the front wheels and extends rearward, (e.g. via a propeller shaft) to the rear wheels. The benefit of all-wheel drive is increased traction which provides increased safety when driving under slippery road conditions and when driving through off-road terrain where two-wheel drive vehicles are likely to get stuck (e.g. in snow or mud).
A second desirable feature is the provision of anti-lock brakes. Anti-lock brakes prevent skidding. Both braking ability and driver control are detrimentally effected when the wheels lock and thus skid on the road surface. Anti-lock brakes may be beneficially applied to both front and rear brakes but the major benefit can be achieved by applying the anti-lock brakes to the rear wheels only.
However, a problem occurs when equipping vehicles with both all-wheel drive and anti-lock brakes. Because the front and rear wheels are coupled together, any torque that is applied to one set of wheels (e.g. the front wheels) is transmitted through the drive train to the other set of wheels (e.g. the rear wheels). In the instance of braking, it may be a serious disadvantage to have the braking torque that is applied to the front wheels transmitted to the rear wheels which can interfere with the anti-lock function.
One clear example of the disadvantage of coupling these features is where the vehicle is equipped with all-wheel drive and the rear wheels only are equipped with anti-lock brakes. A panic induced braking situation may result in the front wheels becoming locked. Whereas the anti-lock brakes would typically prevent locking of the rear wheels, because the drive train is coupled to the front wheels the drive train is forced to retard its rotation and the coupling of the drive train to the rear wheels accordingly retards the rear wheels (perhaps to the point of locking) regardless of the anti-lock braking action applied to the rear wheels. The anti-lock feature is thereby effectively negated.
The present invention accordingly has as an objective to accommodate the anti-lock braking feature by the discriminate decoupling of the front and rear wheel sets during the braking action. That is, the wheels are to be coupled when driving action is desired and decoupled when braking action is desired.
One further concern that needs to be addressed, however, is the desirability of engine braking during a desired deceleration of the vehicle, e.g., coasting down hill. Whereas such engine braking may be desirable for both front and rear wheels, it is considered important that at least one of the wheel sets remain engaged for engine braking. Thus, decoupling of the wheels is intended in the present invention for one wheel set only. If the decoupling is applied to a part-time four-wheel drive, the wheel set that is disengaged from the drive train for driving should also be the wheel set that is decoupled for braking.
It follows that in an all-time all-wheel drive vehicle equipped with rear wheel anti-lock brakes, the rear wheel set should be decoupled from the effect of the front wheels. Whereas engine braking is sacrificed to accommodate the anti-lock braking feature, it is only true for one of the wheel sets and in balance, the combination of features is desirable regardless of the reduction in engine braking.
The present invention is not the first to recognize the desirability of decoupling the drive train between the front and rear wheels on an all-wheel drive vehicle. The Chrysler Corporation explained this desirable feature in an article published in the Oct. 1, 1990 automotive issue of Design News. In that article, Chrysler disclosed a clutch mechanism that was incorporated into the rear propeller drive shaft of an all-wheel drive vehicle. The clutch mechanism caused the rear wheels to be engaged by the propeller drive shaft when the rotative force of the propeller drive shaft was driving the wheels and it provided disengagement when the propeller drive shaft tried to retard the wheels. However as also explained, that same mechanism operated in one rotative direction only. When the vehicle was driven in reverse whereby the drive shaft was rotatively reversed, the mechanism disengaged the rear wheels from the drive axle in the driving mode, a serious drawback for the vehicle when attempting to back out of a "stuck" condition. This problem was addressed and overcome by Chrysler with an auxiliary by-pass mechanism applicable only in the special case of providing driving engagement to the rear wheels with the vehicle driven in reverse. This mechanism is disclosed in U.S. Pat. No. 4,867,260.